The Gibraltar West deposit is located near the eastern margin of the Stikine Terrane west of Granite Mountain in south-central British Columbia. The Stikine Terrane is dominantly oceanic and became amalgamated with the Quesnel Terrane to the east probably during Triassic times. The dominant rock types in the region are metabasalt, limestone and argillaceous metasediments of the Mississippian to Triassic Cache Creek Group. These are intruded by the dioritic to quartz dioritic Late Triassic Granite Mountain pluton and the (?)Cretaceous Sheridan Creek pluton. The Granite Mountain pluton has been dated at 204 +/- 6 Ma by potassium-argon dating of hornblende (CIM Special Volume 15 page 195). Jurassic sedimentary rocks overlap both the Cache Creek and Quesnel terranes to the north and east of the plutons. Older rocks are largely obscured by Plateau Basalt of probable Miocene age, to the west. The Granite Mountain pluton has been affected by regional metamorphism (greenschist facies) and deformation along with the enclosing Cache Creek Group. The Cache Creek Group and the margins of the Granite Mountain pluton record effects of ductile deformation. The main body of the pluton has been cataclastically deformed.
The Gibraltar West deposit is one of the five orebodies that comprise the Gibraltar mine, the others are: Granite Lake (093B 013), Pollyanna (093B 006), Gibraltar North (093B 011) and Gibraltar East (093B 012). The orebodies are hosted by the Granite Mountain pluton with ore mineralization almost entirely confined to the Mine Phase Tonalite portion of the Granite Mountain pluton. The Mine Phase Tonalite appears to form a thin outer shell about the main body of the pluton and contains approximately 30 per cent quartz, 50 per cent saussuritized plagioclase feldspar and 20 per cent chlorite. Varying degrees and types of alteration are present and readily visible in the Mine Phase Tonalite. Economic sulphide mineralization in the Mine Phase Tonalite is usually associated with sericitization and chloritization. The tonalite has been strongly deformed by shearing and mineralization is strongly associated with this deformation. Mineralization is generally accompanied by alteration and is confined to deformational structures. These structures comprise small and large shear zones, foliation planes, short veins and various dilatant structures.
Mineralization consists of pyrite, chalcopyrite, molybdenite, magnetite, bornite, and cuprite. Associated alteration minerals are quartz, sericite, chlorite, epidote, and carbonate. The Gibraltar deposits all show secondary oxidation and secondary enrichment with the formation of chalcocite as coatings and as replacement of pyrite and chalcopyrite.
As a whole, the Gibraltar mineralized system is comprised of numerous structural hosts for economic mineralization ranging from highly mineralized shear zones to complex sets of sheeted shear veins commonly referred to as oriented stockworks.
Production figures indicate that silver and minor gold mineralization is associated with these orebodies (see Gibraltar East (093B 012) for figures). Total measured recoverable reserves in 1988 for all the orebodies were 183.24 million tonnes, grading 0.32 per cent copper and 0.009 per cent molybdenum (Placer Dome Inc., Annual Report 1988). As of December 31, 1992, mineral resources of Gibraltar West were 29,483,500 tonnes of 0.300 per cent copper and about 0.007 per cent molybdenum (CIM Special Volume 46, page 202). Total mineable reserves in 1995 for all ore deposits are 179.0 million tonnes grading 0.297 per cent copper and 0.009 per cent molybdenum (Gibraltar Mines Limited, Annual Report 1995).
Diamond drilling in 1994 tested for deep ore grade mineralization along the southwest side of the Gibraltar West Stage 1 Pit. All eight drillholes confirmed the presence of a relatively deep mineralized zone. This zone is thought to be a part of the much larger Gibraltar North - Gibraltar West mineralized system which has been intersected in various places by diamond drilling along a southeast-northwest strike length of about 2.5 kilometres. Drilling results to date suggest that the system increases in size and grade towards the northwest (Assessment Report 24624). Mine Phase Tonalite was intersected throughout all of the drillholes. The hostrock is variously altered with quartz, chlorite, sericite, epidote and carbonate. Most of the high grade copper mineralization is found to be associated with either chlorite, quartz-chlorite, quartz-sericite, or quartz-chlorite-sericite alteration. This alteration is generally associated with penetrative deformation, and in most cases, the strength of alteration and the amount of mineralization increased with the intensity of deformation. Chalcopyrite and pyrite were observed in all holes along with minor amounts of molybdenite.